The invention pertains to the field of endless chain power transmission systems. More particularly, the invention pertains to chain transmission systems having inwardly opening teeth.
All current automobile piston engines have a camshaft and a crankshaft. The crankshaft transmits the power from the pistons as they move up and down and changes it to a rotating motion. The crankshaft drives one or more camshafts, which open and close the valves in the engine. A camshaft has eccentric lobes or cams: one lobe for each valve, which in turn push a cam follower or valve lifter which opens the valve. This must happen at exactly the right time for each valve. This coordination of the crankshaft with the opening and closing of the valves via the camshaft is called Valve Timing or Camshaft Timing.
While the most direct way of driving the camshaft might be by gears on each meshing with each other, this is usually not practical given the location of the crankshaft at the bottom of the engine and the valves (and, in overhead camshaft (OHC) engines, the camshaft) at the top. In many cases, timing chain systems are used to drive the camshaft(s) of an engine from the crankshaft. A chain is looped around a drive sprocket on the crankshaft, and drives a driven sprocket on the camshaft (or one on each camshaft, in a multiple-camshaft engine).
Even a slight misalignment of the camshaft(s) relative to the crankshaft can adversely affect the performance of the engine, and possibly stop it altogether. In extreme cases, in some engines (called xe2x80x9cinterference enginesxe2x80x9d), if a valve opens all the way at the same time as the piston comes up, the piston will contact the valve. At best, it can bend or break the valve, and at worst, crack the block or otherwise destroy the engine. Even a single tooth""s misalignment can cause this disastrous situation in some interference engines.
This kind of misalignment can be caused by the chain xe2x80x9cjumpingxe2x80x9d a tooth or more, due to slack in the system. FIG. 7 shows how little slack is required for a link (72) in a chain (71) to jump a tooth on a sprocket (70) of the prior art.
As a result, designers of timing chain systems attempt to prevent the chain from jumping teeth on any of the sprockets in the system. Tensioners and guides are frequently used to guide the chain and minimize the slack. Often, ratchet mechanisms are added to tensioners for the purpose of preventing enough slack in a chain strand to allow the chain to rise over and move past a sprocket tooth.
For noise and weight reduction, the teeth on the sprockets and chain pitch in timing chain applications are often reduced in size. As the size of these components is reduced, the potential for the chain xe2x80x9cjumpingxe2x80x9d or slipping in alignment relative to the sprocket is increased, as it takes less slack in a chain strand to move over and past a small sprocket tooth (small pitch) than a larger tooth. The use of small pitch chains is thus prohibited in many cases by the risk of tooth jumping.
A design which would make it more difficult for a chain to jump teeth on a sprocket would be useful for at least two reasons: First, the ratchet mechanism added to the chain tensioner may not be required any longer; Second, the use of very small pitch chains could be considered on applications that currently could not use small pitch chains.
As can be seen in FIGS. 1 and 2, prior art chains are commonly designed with a plurality of parallel inner links (1) which have inward-pointing teeth to engage the teeth of sprockets, connected together with pins (3) to other inner links (2). The inner links are staggered in groups of a number of parallel links followed by a group of one-fewer links, such that if one looked down on a section of chain as shown in FIG. 1, one would see two parallel links (1) pinned (3) to a single inner link (2), then two links, etc. Larger chains might have four inner links pinned to three links, pinned to four links, and so on. Outside guide links (4), which are not toothed, are used parallel to the lesser-number inner link sections (2). The teeth (5) are all of equal width, and engage the inward-extending teeth of the links (1) and (2) (teeth (5) are shaded in FIG. 1 to distinguish them from the links (1)(2)).
Guthrie, xe2x80x9cDrive Chain Gearxe2x80x9d, U.S. Pat. No. 495,584, shows a sprocket and chain system using a modified sprocket which drives the chain between linksxe2x80x94that is, the sprocket teeth contact the outsides of the links, rather than through the teeth on the links as in the chain of the invention. Guthrie uses a sprocket which appears to have two rows of thin teeth in parallel, offset by half-a-tooth. While Guthrie""s arrangement would have a similar effect to the invention, the drive system is very different, and would require very large teeth, rather than being operable with small teeth and pitches as the current invention.
Kirsten, xe2x80x9cPower Transmitting Mechanismxe2x80x9d, U.S. Pat. No. 1,835,406, uses a similar rows of parallel teeth system to Guthrie, although Kirsten""s teeth (or rather the gaps between the teeth) do contact inward-pointing teeth on the chain. Kirsten""s sprocket wheel is made up of many laminated thin sprockets, each lamination offset by a half-tooth from its neighbors.
Martin, xe2x80x9cChain Drive including Sprocket Having Alternate Wide and Narrow Teethxe2x80x9d, U.S. Pat. No. 4,174,642, shows a chain and sprocket arrangement in which the sprocket has alternating wide and narrow teeth, and the roller type chain has alternating wide and narrow gaps within which the teeth fit. The roller-type chain is different from the chain of the inventionxe2x80x94the sprocket drives the rollers, rather than teeth on the links, and the Martin invention would not be suitable for small tooth socketsxe2x80x94small pitch inverted tooth chains as used in the present invention.
Although this invention is described herein in terms of timing chains, which are an important application, it will be understood by one skilled in the art that the invention may also be used in other applications where a small pitch/small tooth chain and sprocket system is desirable.
The invention is a novel chain and sprocket design which is applicable to sprockets which have an even number of teeth and chains which use inward-pointing teeth and parallel links and outer guides.
In the system of the invention, the sprocket is designed such that alternate teeth of the sprocket are of increased width such that when the sprocket is properly oriented they extend outward between outer guide links, which are shaped to allow the teeth to enter the inter-guide gap. If the chain attempts to jump a tooth, such that the sprocket is oriented with the wider teeth aligned with the outer guide links, the wider teeth will contact the guide links and prevent the chain from seating in the sprocket.